Method and apparatus for applying metal coatings to the inner surfaces of metal tubes



Sept. 26, 1961 M. L. ROBINSON 3,001,889

METHOD AND APPARATUS FOR APPLYING METAL commas TO THE INNER SURFACES OF METAL TUBES Filed Feb. 23, 1960 4 Sheets-Sheet 1 III 1 a GIL MY RON ROBINSON ATTORNEYS Sept. 26, 1961 M. ROBINSON METHOD AND APPARATUS FOR APPLYING METAL COATINGS TO THE INNER SURFACES 0F METAL TUBES Filed Feb. 25, 1960 4 Sheets-Sheet 2 T-T \L 7: m .Y IZB L 48 I I |2 m i 3 Z6 I INVENTOR MYRON ROBINGON W f 7 ATTORNEY? Sept. 26, 1961 M. ROBINSON 3,001,889

I METHOD AND APPARATUS FOR APPLYING METAL COATINGS TO THE INNER SURFACES OF METAL TUBES Filed Feb. 23, 1960 4 Sheets-Sheet 3 INVENTOR MYRDN L.ROBIN5ON a 2 ATT I RNE 3,001,889 INGS T0 Sept. 26, 1961 M. L. ROBINSON METHOD AND APPARATUS FOR APPLYING METAL COAT THE INNER SURFACES OF METAL TUBES 4 Sheets-Sheet 4 Filed Feb. 23, 1960 INVENTOR MYRON L. ROUNQON J We,

ATTORNE QTLHI m P24 157. emu

United States Patent 3 001,889 METHOD AND APPARA'IUS FOR APPLYING METAL COATINGS TO THE INNER SUR- FACES OF METAL TUBES Myron L. Robinson, Monterey Park, Califi, assignor to Phelps Dodge Copper Products Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 23, 1960, Ser. No. 10,420

14 Claims. (Cl. 11751) This invention relates to the application of metallic coatings to the inner surface of metallic tubular products, and relates more particularly to an improved method and apparatus for this purpose.

For illustrative purposes, the invention will be described in connection with the application of a coating of tin or tin-lead alloy to the interior of copper or copper-base. alloy tubes furnished in long coiled lengths. However, the invention is also useful for interiorly coating any metal tube with a coating of metal whose melting point is substantially lower than that of the tube metal and which when molten is capable of wetting a suitably fluxed surface of the tube metal to which it is applied. Metalshaving low melting points commonly used in this application are: Tin, lead, tin-lead alloys, zinc, and aluminum, the last two named metals being intended for coating steel tubular products. i

Copper and copper-base alloy tubes are used extensively for plumbing and for carrying gases and-oils used as fuels. In some areas, the water is highly corrosive to copper and its alloys, forming metallic salts which often result in a coloring of the water and forming what is commonly described as green water. This corrosive action eventually leads to failure of the tube by perforation of the tube wall, and it is quite often more notice able in installations such as domestic buildings Where the Water flow is intermittent. Certain gases and oils used for fuel contain hydrogen sulphides and organic sulphur compounds which will form copper sulphide when in contact with copper. The copper sulphide is not formed as a tenacious or continuous film on'the corroded, copper. It breaks off as fine particles from the corroded base metal and is carried along by the fluid.

The green water problem in water lines and formation of non-adherent copper sulphidein gas and oil lines made of copper have been deterrents to the successful use of bare copper tubes for conveying the fluids, where these corrosive conditions are known to exist. In gas or oil lines, some of the hazards due to the presence of nonadherent particles of copper sulphide are:

(1) Contamination of the fluid with the loose copper sulphide particles which lodge or accumulate at some point in the line and thereby'restrictthe flow of the fluid.

(2) Collecting of particles at, and resultant jamming of, delicate control mechanisms on fuel lines, such as thermostatically operated valves on appliances and pilot controls on gas handling systems, causing either interrupted flow or diverging of the flow of the potentially explosive fluids into areas where a destructive explosionmay be set off.

(3) Ultimate failure by corrosion of unprotected copper tube.

It is Well known to those versed in the art that copper and copper-base alloy products coated with a tin or a tin-lead alloy are substantially impervious to the type of corrosive attack described above.

The coating of short straight lengths of tubes with tin or tin-lead alloys by hot-dipping or by electroplating methods is well known in the art. Coating of the inner surface of tubular products by this method, however,

Patented Sept. 26., 1961' ice diameter in lengths generally not in excess of 30 feet, and the present methods used for the deposition of metallic coatings on the inner surfaces of tubes are generally so costly as to preclude their use for coating tubes whose length exceeds 30 feet.

It is an object of this invention to provide an improvedapparatus and method for applying, with a measure of control over thickness, a continuous and adherent coating of metal economically on the inner surface of tubular products in coils or other shape, without coating the outer surface or substantially changing the condition of the base metal. As an example, by this invention the inner surface of coiled tubes with bore diameters as small as inch and 4; inch and lengths exceeding 50 feet and 200 feet, respectively, havebeen successfully coated. The maximum bore size and length limit have not been established, but bore sizes in excess of 1 /2 inches and lengths over 1,000 feet seem practical with the apparatus of suitable capacity.

During the coating of copper and copper-base alloys by continuous hot dipping in a tin bath, an undesirable feature is that copper is dissolved in the molten tin bath, and the increasing concentration of dissolved copper in the molten bath causes an undesirable increase in the temperature required to keep the bath molten and results in.

the formation of a drossy waste sludge high in tin content on top of the bath.' This drossy sludge requires costly reprocessing to reclaim its tin content. i i

It is also an object of the invention to provide a method and apparatus for bringing a controlled amount of molten tin in contact with the interior surface of a copper or copper-base alloy tube under conditions which practically eliminate the expensive dressy waste by-pr0duct normally associated with continuous hot-dip tin coating operations.

In the method according to the'present invention, a coiled length of the tube is inserted in a heating chamber where it is pro-heated to a temperature at least as high as the melting point of the metal to be coated on the interior surface of the tube. A mass of the coating metal is maintained in molten condition, preferably by subjecting it to the heat in the heating chamber; and from this mass is withdrawn a slug of molten metal in excess of the quantity required to coat the interior surface of the tube. A wiper is inserted in the tube, and the slug of molten metal preceded by the wiper and in contact therewith is blown through the tube in the chamber while the tube is at substantially the temperature to which it was preheated. In this way, a film of the coating metal is deposited over the interior surface of the tube. Preferably, the wiper 'is saturated with a flux before it is inserted in the tube, and the wiper and excess molten metal are blown from the tube into the molten mass of coating metal, the slug of molten metal for each coating operation being withdrawn from near the bottom of the mass of molten metal. For more reliable results, the interior surface'of the tube is cleaned and'fluxed before blowing the wiper and molten metal through the tube, and a second wiper'is blown through the tube after blowing the first wiper. and slug therethrough, so as to smooth out the deposited film of coating metal. The tube may then be further treated by passing a washing fluid and then a drying medium through it. i

An apparatus made according to the invention corn prises an oven forming a heating chamber for receiving a coiled length of the tube to be coated interiorly, means forheating the chamber to a temperature at least as high as the melting point of the coating metal, and a container for storing a supply of molten coating metal. k This con tainer may be located in the even so that the aforesaid heating means also serves to maintain the coating metal in a molten condition. A fitting in the oven has a passage adapted for communication at one end with one end of the coiled tube in the chamber, this passage opening at its other end exteriorly of the oven to afford access for a wiper to be inserted through the passage and into the adjacent end of the tube. By means of a quick-opening closure, this outer end of the passage may be closed to permit the blowing operation previously described. A conduit leads from the molten metal container, preferably from near the bottom thereof, to a well in the chamber for receiving a measured quantity of molten coating metal which is in excess of the quantity required to coat the interior surface of the tube. This well communicates with the passage in the fitting intermediate the ends of the passage. A propellant gas supply conduit also leads to the well, and both the latter conduit and the conduit for supplying molten metal to the well are provided with valve means.

In the preferred operation of the apparatus, the oven is pre-heated to melt the supply of coating metal in the container. Then the coiled tube, preferably pre-iluxed, is placed in the oven and one end of the tube is connected to the fitting, as by means of a releasable gripping means, so that the tube communicates with the inner end of the passage in the fitting. Prior to the coating operation, the coiled tube is heated in the oven to the desired coating temperature, which is close to the melting point of the tin or other coating metal. To effect the coating, a wiper, preferably made of a soft, heat-resistant fabric saturated with a suitable flux, is introduced into the fitting passage through the outer end thereof and inserted in the tube; and by operation of the valve means, molten metal is caused to flow from the container into the well until the latter is filled to the required level, after which the outer end of the fitting passage is closed by the removable closure. The valve means are then operated to open the propellant gas supply conduit leading to the well, so that the slug of molten metal is blown from the well through the tube preceded by the wiper. Preferably, the end of the tube through which the wiper and slug are ejected is connected, as through a releasable gripping means, to a fitting which forms a return passage leading into the reservoir or container for the molten metal, so that the excess molten metal is blown into this container along with the wiper. The latter may be recovered from the top of the container along with any flux adhering to the wiper, as through an opening in a wall of the oven.

For a better understanding of the invention, reference may be had to the accompanying drawings illustrating a preferred form of the new apparatus.

In the drawings,

FIG. 1 is a horizontal sectional view of the apparatus, the section being taken near the top of the oven to reveal the interior thereof;

FIG. 2 is a sectional view on the line 22 in FIG. 1;

FIG. 3 is an end view of the oven as seen from the right in FIGS. 1 and 2, the end wall being partly broken away;

FIG. 4 is an enlarged vertical sectional view of part of the apparatus, showing the well connected to the fitting and the releasable clamping means for connecting one end of the coiled tube to the fitting;

FIG. 5 is an enlarged fiagmentary end view of the oven as seen from the right in FIGS. 1 and 2, showing the releasable closure for the outer end of the passage in the fitting;

FIG. 6 is an enlarged fragmentary front elevational view of the container for the molten coating metal, showing the releasable gripping means for securing the outlet end of the coiled tube to the return fitting on the container;

FIG. 7 is a view similar to FIG. 6 exceptthat it is a side elevational view;

FIG. 8 is a longitudinal sectional view of a tube, illustrating the passage of the wiper and slug therethrough incident to the blowing step;

FIG. 9 is a plan view of a wiper suitable for insertion in the tube; and

FIG. 10 is a plan view of a tool which may be used for inserting the wiper in the tube.

Referring to the drawings, particularly FIGS. 13, the illustrated form of the apparatus comprises an insulated oven 12, the inner surfaces of the oven chamber being formed by a reflective metal lining. The front wall of the oven has an opening which is normally closed by an insulated door 12a mounted on hinges 1212, the wall opening being of sufficient size to insert a coiled tube T into the oven. A perforated rack 13 is mounted in the oven in position to support the coiled tube T, and between this rack and the bottom surface of the oven are suitable heating devices 14. The heating devices 14 may be of any well-known type, such as conventional oil or gas burners or conventional electric heaters. As shown, the heating devices 14 are burners which are supplied with fuel through pipelines 14a leading through the bottom of the oven. Small fans 15 are located in the oven below the rack 13 and adjacent one or more side walls of the oven, these fans serving to increase the circulationof the hot gases inside the oven. A thermometer 16 is mounted on the top of the open and serves to indicate the oven temperature.

A container 18 for storing a supply of molten coating metal is mounted in the upper portion of the oven chamber on a bracket 18a. The reservoir formed by the container 18 is closed at the top by a removable cap 19 having a vent tube 20 which extends upwardly through an opening in the top wall of the oven. By removing the cap 19 through the oven opening 12c, the container 18 may be charged with a supply of the metal with which the inner surface of the tube T is to be coated.

A conduit 22 leads from near the bottom of container 18 to a well 23 which is releasably secured to a fitting 24 in the oven, as by means of bolts 25. The fitting 24 is of generally cylindrical shape, and the open upper end of Well 23 is configurated to fit tightly against the lower portion of the cylindrical surface of the fitting. As shown particularly in FIGS. 4 and 5, the fitting 24 extends through an opening in an end wall of the oven 12 and is provided at its outer end with a flange 26 engaging the outer surface of the oven wall. By means of bolts 27 extending through the flange 26, the fitting 24 is releasably secured to the oven wall.

The fitting 24 has a longitudinal passage 28 (FIG. 4) which communicates with .the well 23. More particularly, a bottom opening 29 in the fitting leads upwardly from the well 23 to the fitting passage 28 intermediate its ends, the fitting being recessed interiorly at 30 to provide in efiect an enlargement of the opening 29.

At its inner end, the passage 28 of the fitting is flared to form a frusto-conical recess 31 for receiving one end of the coiled tube T. The outer end of passage 28, where it opens through the oven wall, is adapted to be sealed by a door 32 mounted on a hinge 33 secured to the outer surface of the oven wall. The door 32 carries a resilient closure member 34 which, in the closed position of the door, protrudes into the passage 28 and effectively closes the outer end thereof. By means of a latch 35, the door 32 is held in its closed position. Thus, member 34 constitutes in effect a releasable closure for sealing the outer end of passage 28.

A conduit 37 is connected to a source of propellant gas (not shown) located outside the oven, and this conduit leads from the gas source through the bottom of oven 12 to the well 23, the conduit opening into the well through the bottom thereof. By means of a valve 38, the well 23 may be connected to or disconnected from the propellant gas source. The latter may consist of a closed vessel charged under pressure with .air, carbon dioxide,

sporsss outside the oven by means of an operating handle 40..

As previously mentioned, one end of the coiled tube T is inserted in the flared recess 31 at the inner end of fitting 24. In order to hold this end of the tube tightly in the fitting 24, a releasable gripping means of any suitable type is provided. As shown particularly in FIGS. 4 and 5, such gripping means comprises an arm 42 mounted at its lower end on a pivot 43' fixed in the oven, whereby the arm extends upwardly opposite the flared inner end of the fitting 24. Intermediate its ends, the arm 42 has an opening 42a through which the tube T is adapted to extend. A strip of spring metal 44 is welded or otherwise secured to the arm 42 intermediate its ends and has a free end portion 44a which is olfset from the plane of the arm 42 in the direction of the fitting 24. This free end portion 44a has a hole 4412 through which the tube T is adapted to extend, the upper edge of the hole presenting a sharp edge which is adapted to bite into the exterior surface of the tube. At its upper end, the arm 42 is pivotally connected to a rod 45 extending through an opening 12a. in the oven wall, the outer end of the rod being provided with an operating handle 46. A detent 47 is secured to the oven wall adjacent the opening 12d and is formed to fit into'a notch in the rod 45 so as to hold the arm 42 in a vertical position wherein the end of tube T is seated firmly against the flared surface 31 of fitting 24. By raising the handle 46 slightly, the detent 47 is disengaged from the notch in rod 45, so that the latter may be pushed inwardly to swing the arm 42 to the tube-release position shown in dotted lines in FIG. 4. V

In the operation of the releasable tube-gripping means 4247, the lower end portion of the coiled tube T is inserted through the aligned holes 42a and 44b when the arm 42 is in the retracted position shown by dotted lines in FIG. 4, so that the sharp upper edge of the hole 44b cuts into the tube surface. When it is desired to move the tube end into position for communicating with the fitting passage 28, the rod 45 is drawn outwardly by handle 46, thereby swinging the arm 42 to its vertical position. wherein the tube end is seated firmly against the .flared surface 31 and aligned thereby with the tube passage 28, the parts being locked in this position by engagement of detent 47 in the notch in rod 45. Because of the spring action of strip 44, the latter is adapted to yield somewhat after the tube end has been seated on the flared surface 31 and before the rod 45 hast-been drawn outwardly enough to engage its notch with detent 47.

As shown particularly in FIGS. 6 and 7, the container 18 for the molten coating metal is provided at its upper portion with a hollow boss 49 forming a central passage 52 communicating with the interior of the container. The boss49 receives a bushing 51, which may be releasably secured in the boss in any suitable manner, as by a press fit. The bushing 51 forms a central passage 50 which receives the end portion of the tube T with a loose fit and is flared at its outer end. At its outer end, the bushing has a shoulder or flange 53 seated against the outer end of the boss 49. A plate 55 is hinged at 55a on a bracket secured to the boss 49 and is provided at its free lower end with a sharp edge 55b. The plate 55 may be swung on its hinge 55a by means of a handle 56.

The parts 5556 form a quickly releasable tubegripping means for preventing the upper end of tube T from slipping out of the passage 50 in bushing 51. In operatiomthe upper end of tube T is inserted into the bushing passage 50, after which the sharp edge 55b of plate 55 will cut into the tube due to the weight of handle 56. By swinging handle 56 to the right (FIG. 7), the holding plate 55 is raised from the tube T, thereby releasing the latter so that it can be withdrawn from bush ing 51.

' It will be understood that the form of tube-gripping means 55-56 associated with the container 18 for the molten coating metal may be substituted for the form of gripping means 43-47 associated with the well fitting 24. Also, it will be understood that other forms of tube-gripping means may be used in place of those illustrated, as the invention does not depend upon or reside in any particular form of such gripping means. The tube-gripping means should hold the tube ends so that flux and tin do not escape into the heating chamber to spoil the appearance of the exterior surface of the tube. 5 In the present practice of the invention, the coiled tube T is first placed on a work rack (not shown), where it is prepared for the coating operation by flowing a small quantity of liquid cleaner containing flux into the tube. This is followed by blowing through the tube a wiper consisting of a soft piece of waste cloth saturated with a flux, so as to elfect complete wetting of the inner surface of the tube. This wiper is blown through the tube by connecting one end thereof to a compressed air line or other source of gas under high pressure. The flux used for saturating the wiper may be any of those commonly used in the art of hot-dip coating, such as an aqueoussolution of zinc chloride, ammonium chloride,

Glacial acetic acid 5 Concentrated hydrochloric acid 20 Fluxing agent 15 Methanol 60 The fluxing agent used in the above solution may, for example, be prepared by dissolving 32 oz. of zinc chloride, 12 oz. of ammonium chloride, and 1 oz. of hydrochloric acid in sufiicient methanol to make up one gallon of solution. The methanol provides a non-oxidizing vapor which helps expel air when heated.

After pre-heating the oven 12 to melt the supply of coating metal in the reservoir or container 18, the, coil of fluxed tubing T is inserted in the oven on the supporting rack 13 through the access opening provided by the door 12a. The lower end of the tube is then secured against the flared surface 31 of fitting 24 by the gripping means 4247, as previously described; and the upper end of the tube is secured in the bushing 51 by the gripping means 5556, as previously described. The. oven door 12:: is then closed and the oven is brought toa temperature of approximately 500 F. to heat the coiled tube T to the desired coating temperature (approximately 500 F.) Then a wiper 58 (preferably fiber-glass), saturated with a suitable flux, is formed into a wad and inserted into the lower end of tube T through the passage 28 in fitting 24, the door 32 being opened to permit this insertion. The flux for saturating. the wiper may be of any conventional type used for wetting copper surfaces incident to hot-dip coating. A suitable form of the wiper is shown at 58 in FIG. 9, and it may be inserted through the fitting 24 by means of a rod 59 shown in FIG. 10. The valve 39 is then opened to allow molten metal to tfiow from'the bottom of reservoir 18 into the Well 23 until the surface of the molten metal rising in the well is visible from outside the oven through the fitting passage 28. The valve 39 -and the door 32 are then closed to stop the flow of molten metal to well 23 and seal the outer end of fitting passage 28.

The apparatus is now ready for the coating operation per se, which is effected by opening the propellant gas valve 38 rapidly. This causes the compressed propellant gas in the valved conduit 37 to act on the bottom of the slug of molten metal in well 23, forcing the slug upwardly through opening 29, inwardly through fitting passage 28 (FIG. 4), and into the lower end of the coiled tube T. Thus, the slug of molten metal, preceded by the wiper formed by the flux-saturated fabric 58, is driven through the full length of the coiled tube T, the wiper and excess molten metal being ejected from the upper end of the tube and blown through the fitting 51 and boss 59 into the upper portion of the reservoir or container 18. The completion of the coating operation is signaled by a slightly explosive noise indicating passage of the wiper into the container 18, at which time the propellant gas valve 38 is closed.

The passage of the wiper and the slug of molten metal through the tube T, under the blowing action of the propellant gas, is illustrated in FIG. 8. As there shown, the slug of molten metal S, as it is driven to the left through the tube T by the blast of propellant gas directly behind the slug, is preceded by and directly in contact with the wiper 58. Thus, the wiper not only serves to wipe the inner surface of the tube and apply the flux solution to this surface in advance of the slug 5, but also confines the molten metal behind the wiper so that the metal is maintained as a slug completely filling the space directly behind the wiper, whereby the entire inner surface of the tube receives a thin coating or film 60 of the molten metal.

Upon completion of the coating operation and closing of propellant gas valve 38, as previously described, the metal coating 60 on the inner surface of the tube may be subjected to a smoothing operation. This is effected by re'opening the small door 32, inserting another fluxed wiper into the lower end of the tube through the fitting passage 28, closing and latching the door 32, and reopening the valve 38 to blast this wiper through the tube T by means of the propellant gas. Because the tube T and coating metal are still at an elevated temperature in the oven, the blowing of the second wiper through the tube serves effectively as a final wiping operation to smooth out the surface of the coating metal 60. When this second wiping operation has been completed and the gas valve 38 closed, the oven door 12a is opened and the coiled tube T is removed from the oven, after disconnecting the ends of the tube from the fitting 24 and bushing 51, respectively, through operation of the releasable gripping means 42-47 and 55-56, as previously described. In the event that it is desired to provide the inner surface of the tube with a heavier or more uniform metal coating than that obtained by a single pass of the coating metal, the operation may be repeated one or more times before disconnecting the ends of the tube from the parts 24 and 29, respectively.

It will be understood that the fabric wiper or wipers 58, being of much lower specific gravity than the coating metal, will float on top of the supply of coating metal in the reservoir or container 18 and may be readily withdrawn from the latter by removing the cap 19 through the oven opening 12c. Also, since the molten metal flowing by gravity to the well 23 is drawn from near the bottom of the reservoir 18, it will not be contaminated by any oxides or sludges, as any oxidesor sludges accumulating in the reservoir 18 will float on the molten metal therein and in part adhere to the absorbent wipers. Furthermore, by returning the excess coating metal ejected from the tube T to the molten metal reservoir 18, an obvious economy is obtained.

A further advantage of the present invention is that upon removal of each coated tube T from the oven 12, another coiled tube may be inserted in the oven and similarly treated, the reservoir 18 providing an adequate supply of molten coating metal for a number of coating operations and being easily re-charged through the oven opening 120. Additionally, the apparatus maybe readily adapted to handle tubing of different lengths and internal diameters. That is, if it is necessary to increase or decrease the quantity of molten metal required for the coating operation, the well 23 may be replaced by a larger or smaller well, since it is releasably connected to the fitting 24 by the bolts 25 and may be releasably connected to the propellant gas conduit 37 by a conventional releasable coupling (not shown). This feature enables selection and control of the amount of molten metal to be blown through the tube during the coating operation. Similarly, the fitting 24 may be replaced by a fitting for accommodating a tube or larger or smaller diameter, since it is releasably secured to the oven by the bolts 27; and the fitting or bushing 51 may be replaced by one for accommodating a tube of different diameter.

It is highly desirable that the well 23 be dimensioned to hold a slug of molten tin only slightly in excess of the quantity needed for coating the tube T. Among the advantages of this is the fact that a small slug can be moved faster at lower temperature without excessive pressure, and less copper can be dissolved by the slug in the coat ing operation. Also, a smaller slug discharging from the coil is easier to control.

After the coiled tube T has been coated interiorly and removed from the oven 12, the tube may be washed and rinsed by flowing water or wet steam through the tube and dried by driving one or more waste cloth wipers through the tube by compressed air, followed by hot dry air or gas.

I claim:

1. A method of coating the interior surface of a metal tube with a coating metal having a melting point substantially lower than that of said tube, which comprises inserting a coiled length of the tube in a heating chamber and there pro-heating the tube to a temperature at least as high as the melting point of said coating metal, maintaining a molten mass of said coating metal, withdrawing from said mass a slug of molten metal in excess of the quantity required to coat said interior surface, inserting in the tube a porous wiper satuarted with a liquid adapted to treat said interior surface for coating with said coating metal, and blowing the slug preceded by the wiper and in contact therewith through the tube in said chamber while the tube is at substantially said temperature, thereby depositing first said treating liquid and then a film of the coating metal on said interior surface.

2. The method according to claim 1, in which said mass of coating metal is maintained molten by subjecting it to the heat in said chamber.

3. The method according to claim 1, comprising also blowing the wiper and excess metal from the tube into said molten mass.

4. The method according to claim 1, in which said slug of molten metal is withdrawn from the bottom of said mass of molten metal.

5. The method according to claim 1, comprising also blowing a second wiper through the tube following said first Wiper and slug, thereby smoothing out the deposited film of coating metal.

6. The method according to claim 1, comprising also blowing a second wiper through the tube following said first wiper and slug, and while the tube is still in heated condition, thereby smoothing out the deposited film of coating metal.

7. The method according to claim 1, comprising also passing first a washing fluid and then a drying medium through the tube after said blowing step.

8. The method according to claim 1, comprising also cleaning and fluxing the interior surface of the tube prior to inserting the tube in the heating chamber, blowing the wiper and excess molten metal from the tube into said molten mass, said slug of molten metal being withdrawn from the bottom of said mass, and blowing a second wiper through the tube following said first wiper and slug and while the tube is still in heated condition, thereby smoothing out the deposited film of coating metal.

9. Apparatus for applying to the interior surface of a metal tube a coating of a metal having a melting point la 9 lower than that 01 the tube metal, which comprises an oven forming a heating chamber for receiving said tube, a container for storing a supply of molten coating metal, means for heating said chamber to a temperature at least as high as the melting point of the coating metal, a fitting in the oven having a passage adapted for communication at one end with one end of a tube in the chamber, the passage opening at its other end exteriorly of the oven to afford access for a wiper to be inserted in said passage and tube end, a releasable closure for said other end of said passage, a well in the chamber for receiving a measured quantity of molten coating metal and communicating with said passage intermediate said ends thereof, a metal supply conduit leading from said container to the well, a propellant gas supply conduit leading to the well, and valve means in said conduits for admitting a charge of molten metal into the well while cutting off the supply of propellant gas to the well, the valve means being adjustable to admit said gas to the.well while disconnecting the well from said container, whereby the apparatus is operable to blow molten metal from the well through the tube preceded by said wiper.

10. Apparatus according to claim 9, in which said container is located in the oven.

11. Apparatus according to claim 9, in which said metal supply conduit leads from the bottom of the container.

12. Apparatus according to claim 9, comprising also a fitting adapted to engage the other end of the tube and forming a return passage leadinginto the container.

13. Apparatus according to claim 9, comprising also releasable means for holding said one end of the tube in engagement with the fitting.

14. Apparatus according to claim 9, comprising also a fitting adapted to engage the other end of the tube and forming a return passage leading into the container, and releasable means for holding said ends of the tube in engagement with the respective fittings.

References Cited in the file of this patent UNITED STATES PATENTS 938,489 James Nov. 2, 1909 1,818,008 Ritter et a1 Aug. 11, 1931 2,470,689 Crampton et al. May 17, 1949 2,480,358 Curtis et al. Aug. 30, 1949 

1. A METHOD OF COATING THE INTERIOR SURFACE OF A METAL TUBE WITH A COATING METAL HAVING A MELTING POINT SUBSTANTIALLY LOWER THAN THAT OF SAID TUBE, WHICH COMPRISES INSERTING A COILED LENGTH OF THE TUBE IN A HEATING CHAMBER AND THERE PRE-HEATING THE TUBE TO A TEMPERATURE AT LEAST AS HIGH AS THE MELTING POING OF SAID COATING METAL, MAINTAINING A MOLTEN MASS OF SAID COATING METAL, WITHDRAWING FROM SAID MASS A SLUG OF MOLTEN METAL IN EXCESS OF THE QUANTITY REQUIRED TO COAT SAID INTERIOR SURFACE, INSERTING IN THE TUBE A POROUS WIPER SATURATED WITH A LIQUID ADAPTED TO TREAT SAID INTERIOR SURFACE FOR COATING WITH SAID COATING METAL, AND BLOWING THE SLUG PRECEDED BY THE WIPER AND IN CONTACT THEREWITH THROUGH THE TUBE IN SAID CHAMBER WHILE THE TUBE IS AT SUBSTANTIALLY SAID TEMPERATURE, THEREBY DEPOSITING FIRST SAID TREATING LIQUID AND THEN A FILM OF THE COATING METAL ON SAID INTERIOR SURFACE. 